In one or more embodiments, this disclosure is directed to a system and method useful for generating and processing automatic repeat request (ARQ) signals in wireless communications networks. In particular, this application is directed to a system and method for dynamic control of ARQ signals in wireless communications networks that involve a hybrid approach over conventionally known signal protocols and communications standards. Even more particularly, this application is directed to a system and method for improving communication channel performance by the dynamic control of hybrid ARQ (HARQ) in Broadband Wireless Access (BWA) communications systems based upon monitoring and selectively responding to changing communication channel conditions to improve data throughput and/or the number of users that may access the communications network.
As a result of the demand for longer-range wireless networking, the IEEE Standard 802.16 was developed. The IEEE 802.16 standard is often referred to as Wireless Metropolitan Area Network (WiMAX), “mobile WiMax”, or less commonly as WirelessMAN or the Air Interface Standard. This standard provides a specification for fixed broadband wireless metropolitan access networks (“MANs”) that use a point-to-multipoint architecture. Such communications can be implemented, for example, using orthogonal frequency division multiplexing (“OFDM”) communication. OFDM communication uses a spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the “orthogonality” that prevents the demodulators from seeing frequencies other than their own. Expected data throughput for a typical WiMAX network is 45 MBits/sec per channel. The 802.16e standard defines a media access control (“MAC”) layer (OSI level 2, sometimes referred to as the “Radio Link Control” or “RLC” layer) that supports multiple physical layer specifications customized for the frequency band of use and their associated regulations. This MAC layer uses protocols to ensure that signals sent from different stations using the same channel do not interfere with each other or “collide”.
The IEEE 802.16 system architecture, for example, consists of two logical entities, the Base Station (BS) and the Subscriber Station (SS). Both the BS and SS have instances of the IEEE 802.16 MAC and Physical Layer 1 (PHY), in addition to other support functions. However, specific functions performed by the MAC or PHY differ depending on whether it is a BS or SS, and the IEEE 802.16 standard defines the BS- and SS-specific behavior in detail. The term SS is applied in a fixed context, while the MS is used in a mobile environment, as introduced by IEEE Std 802.16e.
In Point-to-Point (PtP) and Point-to-Multipoint (PMP) networks, the BS and SS are in a master-slave relationship, where the SS must obey all medium access rules enforced by the BS. The mobile station (MS) defined in the IEEE 802.16 mobility extension (IEEE Std 802.16e) requires support for additional SS-specific functions such as mobility management, handoff, and power conservation. In this disclosure, the term “SS” is intended to not only include fixed or relatively immobile terminal equipment, but to also include MS functionality of mobile user terminal equipment, unless specifically stated otherwise. One of the basic differences between the BS and SS in a PMP network configuration is that the BS, which acts as a centralized controller and a centralized distribution/aggregation point, has to coordinate transmissions to/from multiple SSs, whereas the SS need only to deal with one BS. All traffic originating from an SS, including all SS-to-SS traffic must go through the BS. Therefore, in a typical IEEE 802.16 system, the BS has to have additional processing and buffering (i.e., memory) capability in comparison to a typical SS to support a reasonable number of SSs.
The functions of the BS and SS depend on the operation mode, namely, PMP or mesh. The functions of the Base Station include:                Enforce MAC and physical parameters such as frame size.        Perform bandwidth allocation for downlink and uplink traffic per SS.        Perform centralized Quality of Service (QoS) scheduling based on the QoS parameters configured by the management system and the active bandwidth requests received from the SS.        Transmit/receive data and control information to/from one or more SSs.        Provide SS support services like ranging, clock synchronization, power control and handover.        
The functions of the Subscriber Station or Mobile Station include:                Identify the BS, acquire physical synchronization, obtain MAC parameters and join the network.        Establish basic connectivity, setup data and management connections and negotiate parameters as needed.        Generate bandwidth requests for connections.        Receive all scheduling and channel information broadcasted and proceed according to the medium access rules provided by the BS, unless in sleep mode.        Perform specific functions for mobility management, handover and power conservation.        
Various methods and metrics have been developed to indicate the channel condition or Channel Quality Indicator (CQI). Exemplary metrics include a Physical Carrier to Interference plus Noise Ratio (PCINR), a Received Signal Strength Indicator (RSSI), an ACK/NACK ratio that indicates a proportion of successful data transmissions to unsuccessful transmission (thereby indicating channel stability), PCINR Standard Deviation (SD) that may indicate Doppler and fading effects that result from movement of the MS, and other indicators. These indicators may be generated at the MS or SS and transmitted to the BS by known techniques of representing the CQI. The BS may receive the channel condition indicators, e.g., CQI, and attempt to adjust communication in response to changes to the channel condition. For example, the BS may perform download link adaption such as, for example, selecting an appropriate Modulation Coding Scheme (MCS) according to the channel condition in response to various changes to the channel condition. Knowing current and accurate channel condition information may enhance the ability of the BS to respond to changes to the channel condition. Current systems, however, are not configured to make use of such channel condition information to change various signaling parameters, particularly in an automated fashion.
Manual selective implementation of a well-known error control technique for data transmission, Automatic repeat-request (ARQ), utilizes acknowledgments and timeouts to achieve reliable data transmission. ARQ acknowledgments are messages sent by the receiver to the transmitter to indicate that the receiver correctly received an information unit. Timeouts are reasonable points in time after the sender transmits the information unit. The sender usually re-transmits the information unit if it does not receive an acknowledgment before the timeout. It continues to re-transmit the information unit until it either receives an acknowledgment from the receiver or exceeds a predefined number of re-transmission attempts. FIG. 2 depicts a notional timeline and message flow for packet transmission/acknowledgement and retransmission in cases where no acknowledgement (negative acknowledgement or NACK) of a transmitted packet is received.
Conventional types of ARQ protocols include “stop-and-wait ARQ”, “go-back-N ARQ” and “selective repeat ARQ”. These protocols typically reside in the Data Link or Transport Layer 2 of the OSI 7-layer model.
Conventional Hybrid ARQ (HARQ) is a commonly used extension of the ARQ error control method that exhibits better performance, particularly over wireless channels, but at the cost of increased implementation complexity. HARQ is used in several conventional wireless communications systems including High-Speed Downlink Packet Access (HSDPA) and High-Speed Uplink Packet Access (HSUPA) (i.e., third generation mobile telephony communications protocols in the High-Speed Packet Access (HSPA) family) which allow networks based on Universal Mobile Telecommunications System (UMTS) to have higher data transfer speeds and capacity on downlink and uplink, respectively, for mobile phone networks using the UMTS.
HARQ has also been used in the currently implemented IEEE 802.16-2005 standard for mobile broadband wireless access, i.e., “mobile WiMAX”. Presently, HARQ provides an important technology for increasing data transmission reliability and data throughput in mobile communication systems. Specifically, in the WiMax implementation, HARQ refers to a combination of ARQ and PHY layer reception techniques like Forward Error Correction (FEC) and signal combining techniques. Different from ARQ operating solely at the MAC layer, HARQ allows the receiver to perform soft-combining of retransmitted packets and therefore may provide some measure of improvement in spectral efficiency. There are two well-known HARQ techniques: the first known as Incremental Redundancy (IR) and the second known as chase combining, discussed further below.
HARQ is an important technique for link adaptation, and makes aggressive modulation and coding schemes (MCS) decisions possible, e.g., the use of OFDM. Thus, the use of HARQ can result in considerable increased data throughput, and/or can enable more users to access the network. In HARQ, the transmitter and the receiver cooperate on an information unit (HARQ sub burst, burst, packet or block) level. The receiver is capable of indicating successful (via ACKs) or unsuccessful (via NACKs) reception of the last transmitted information unit or block. The transmitter comprises several parallel HARQ sub processors (e.g., in 802.16e referred to as HARQ sub-channels), each of which performs operations of transmitting user information units, receiving ACK/NACK information or other ACK indications in response, and performing either a retransmission when needed or transmitting the next information units. The ACK indication may be direct whereby a specific ACK or NACK indication is sent. In HARQ, the receiver takes advantage of any previous retransmissions by decoding the information unit or block based on information gathered from all the retransmissions of the same information unit or block, thus improving overall performance of the communications link.
In IEEE 802.16e, HARQ schemes are optional parts of the MAC layer, and can currently only be enabled on a per-terminal per connection basis, when a Service Flow (SF) is established between the BS and SS. The per-terminal HARQ and associated parameters are specified and negotiated during the initialization procedure, and currently cannot be altered for an established SF. In other words, once HARQ is enabled, it may not be changed during the duration of the particular SF.
As mentioned above, Chase Combining is used in the current WiMAX profile, although IEEE 802.16e also supports IR. A SS may support IR, while a MS may support either Chase Combining or IR. For IR, the PHY layer will encode the HARQ packet generating several versions of encoded subpackets. Each subpacket is uniquely identified using a subpacket identifier (SPID). For Chase Combining, the PHY layer encodes the HARQ packet generating only one version of the encoded packet. As a result, no SPID is required for Chase Combining HARQ Chase Combining requires all retransmissions to send the exact same information and to use the original modulation-coding scheme (i.e. waveform). Note that HARQ retransmissions are asynchronous, in the sense that all HARQ bursts undergo opportunistic scheduling. The maximum number of retransmissions is determined by target residual Packet Error Rate (PER). Typically the number of HARQ retransmissions is set to four, for a PER of 1×10−4 (this is the case for IR as well).
A benefit of employing HARQ is that it can be used to mitigate the effects of channel and interference fluctuation. HARQ provides an improvement in performance due to the SNR improvement derived from the energy and time diversity gain achieved by (1) combining retransmitted packets with previous erroneously decoded packets and/or (2) using Incremental Redundancy (IR) to realize additional coding gain.
Using WiMAX as an example, a resource region for HARQ ACK channels is allocated using the HARQ ACK region allocation Information Element (IE). This resource region may include one or more ACK channels for HARQ support-enabled MSs, e.g., ACKCH 150n in FIG. 1. The uplink (UL) ACK channel occupies half a slot in the HARQ ACK channel region, which may override the fast feedback region. This UL ACK channel is assigned implicitly to each HARQ-enabled burst, according to the order of the HARQ-enabled downlink (DL) bursts in the DL-MAP. Thus, using this UL ACK channel, SSs or MSs can quickly transmit ACK or NACK feedback for DL HARQ-enabled packet data.
HARQ may also divide into several types. In the simplest version of HARQ types, called Type I HARQ, both Error Detection (ED) and Forward Error Correction (FEC) information to each message prior to transmission. When the coded data block is received, the receiver first decodes the error-correction code. If the channel quality is sufficient, all transmission errors should be correctable, and the receiver can obtain the correct data block. If the channel quality is bad and not all transmission errors can be corrected, the receiver detects this situation using the error-detection code, the received coded data block is discarded, and the receiver requests retransmission. The more retransmissions that are required for successful reception, the less are the available resources (e.g., transmission power, number of available transmission slots) to provide data throughput for other users.
In the more sophisticated Type II HARQ, only (1) ED bits or (2) FEC information and ED bits are sent on a given transmission, typically alternating on successive transmissions. It is important to note that detection typically adds only a few bytes to a message, resulting in a relatively small incremental increase in message length. FEC, however, adds error correction parities, which often double or triple the message length. In terms of throughput, standard ARQ typically expends a few percent of channel capacity for reliable protection against error, while FEC ordinarily expends half or more of all channel capacity for channel improvement.
In Type II HARQ, the first transmission contains only data and error detection. If it is received in error, the second transmission includes FEC parities and error detection information. If the second transmission is received in error, error correction is attempted by combining the information received from both transmissions. Incorrectly received coded data blocks are often stored in buffer memory at the receiver rather than discarded. When the retransmitted block is received, the two blocks are combined, using a technique known as Chase Combining, which increases the likelihood of correctly decoding the message.
FIG. 1 depicts the architecture of a WiMAX network implemented in accordance with various aspects of IEEE Standard 802.16. In FIG. 1, base station (BS) 110 may communicate with one or more Mobile Stations/Subscriber Stations (MS/SS) 130a-130n over network 120 via an associated communication channel 140a-140n. In this disclosure, the terms “SS” and “MS” are used interchangeably, although it is recognized that MS implies the use of mobility enhancements. MS/SS 130a-130n may be relatively fixed or immobile terminal equipment, or may be equipment that includes the mobility functions of a MS, e.g., a cell phone or laptop computer traveling in an automobile or airplane. Various factors such as the existence of ambient interference around the SS or BS, movement of the SS, and other factors may degrade or otherwise alter the channel condition of the communication channel, making the use of HARQ desirable to ensure reliable communications over channels 140a-140n. HARQ uplink Acknowledgement Channels (ACKCH) 150a-150n allow each MS/SS 130a-130n to acknowledge packet receipt to the BS by use of a HARQ signal transmission over a dedicated HARQ ACK channel. Although HARQ provides advantages under some channel conditions, these ACKCH channels represent additional channel overhead that will decrease data throughput when channel conditions are such that communication may be reliably maintained without HARQ being used. Channel Quality Indicator (CQI) channels 160a-160n provide a path for the MS or SS to identify the relative quality of the communication channel to BS 110 using known techniques.
FIG. 2 depicts an example of conventional ARQ operation in which each packet is a MAC layer packet containing one or more ARQ blocks. MAC layer ARQ alone does not improve spectral efficiency. However, with its retransmission mechanism to correct packet errors at the cost of extra delays, ARQ provides a more reliable link layer as seen by applications, and permits link adaptation for higher spectral efficiency. As can be seen in FIG. 2, there may be some inefficiency when an ACK is lost due to error, and a correctly received packet is retransmitted.
Chase Combining HARQ in the PHY layer is supported to further improve the reliability of a retransmission stored in a HARQ buffer by combining one or more previous transmissions decoded in error. In HARQ Chase Combining, all retransmissions sent include the same information and use the original modulation-coding scheme (MCS). To streamline the HARQ feedback, a dedicated ACK channel is also provided on the transmission side for purposes of HARQ ACK/NACK signaling, e.g. ACKCH channels 150a-150n in FIG. 1.
In the case of WiMax (or other communication methodologies), the system may, from time to time, encounter communication channel conditions that result in or near complete packet reception, making the use of HARQ or other signaling protocols unnecessary, at least for a portion of the time the communication channel is in use for a particular SF. In this situation, the overhead associated with HARQ signaling (e.g., ACKCH 150a-150n in FIG. 1) must still be allocated because, in conventional systems known to date that implement a HARQ signaling scheme, e.g., WiMax, HARQ may only be enabled at the time a SF is established between a BS and SS, and there is no method or system that solves the problem of the increased overhead and decreased data throughput when HARQ may not be necessary for reliable communications.
Under the current WiMax standard, HARQ and associated parameters are specified and negotiated using Station Basic Capability-Request/Response (SBCREQ/RSP) messages during network entry or re-entry procedure. Under the Standard, a MS shall support per-connection based HARQ, and HARQ can be enabled on a per Connection ID (CID) basis by using Dynamic Service Addition/Dynamic Service Change (DSA/DSC) messages.
HARQ may be enabled or disabled by setting the “ACK disable” bit. HARQ is enabled if “ACK disable”=0, and is disabled otherwise, i.e. “ACK disable”=1. The DL HARQ sub-burst Information Element (IE) defines the “ACK disable” bit setting. However, it may be difficult to determine whether HARQ should be enabled or disabled during initial channel setup, and HARQ may not continue to be necessary if the communication channel's quality improves to an acceptable or desirable level.
A further problem with the current standards-based WiMAX implementation is that the HARQ Enable/Disable decision is fixed and static. The decision on HARQ Enable/Disable is made only when a Service Flow (SF) is setup. Once HARQ is enabled or disabled at the time the SF is established, the communication channel continues with the same HARQ configuration, i.e., no change during SF, regardless of the relative channel condition. Thus, even when channel quality is sufficient for a particular QoS requirement without HARQ, HARQ will continue to be used, resulting in increased network overhead.
Thus, even though conventional HARQ often serves useful purposes, there is a need for a system and method that are capable of enabling network elements in a communication link to dynamically select HARQ processing after a SF has been established to account for the availability of a good communication channel for which HARQ is not required at all times.
Further, the currently-implemented WiMAX implementation has difficulty in making a decision whether HARQ should be enabled or disabled when a new SF is setup since there generally is not enough information about the channel condition for a new SF. Currently, WiMAX always configures HARQ to be enabled in order to minimize the risk of low throughput on the downlink (DL) with the cost of a UL resource for ACKCH. Moreover, the number of Acknowledgement Channels must be increased with the number of users that are enabling HARQ, leading to decreased efficiency in terms of bandwidth utilization and increased overhead.
While the current 802.16 WiMAX standard states that ACK disable bit is configurable to be either a “1” or “0”, the current implementation of the standard provides no guidance concerning when and under what circumstances the ACK disable bit should be set to either a “1” or “0”. As it currently is implemented, any mechanism may be applied, which results in various implementation issues for a large number of stakeholders seeking to widely implement WiMAX.
What is needed for improved channel utilization and increased efficiency in WiMAX networks is a dynamically selectable HARQ Enable/Disable scheme that uses information on changing channel conditions to determine whether HARQ should continue to be enabled or disabled during an ongoing SF by selectively being able to set the “ACK disable” bit.